Automatic dose computer



May 21, 1963 R. G. BAKER ETAL AUTOMATIC DOSE COMPUTER Filed Feb. 26,1960 3 Sheets-Sheet 1 w ST@ May 21, 1963 R. G. BAKER ETAL AUTOMATIC DOSECOMPUTER 3 Sheets-Sheet 2 Filed Feb. 26. 1960 FIG2C1.

In v ew [5v-5 May 21, 1963 R. G. BAKER ETAL AUTOMATIC DOSE COMPUTER I5Sheets-Sheet 5 Filed Feb. 26, 1960 FIGA.

FIGB.

United States Patent @ddee Patented May 2l, 1963 Radiotherapy involvesthe exposure of a patient to a source of radiation in such manner thatthe radiation falls upon a selected locality of the patients body. Thedose of radiation that the patient should absorb at this selectedlocality is prescribed by the radiotherapist according to the nature ofthe condition being treated.

1n most radiotherapy centres the required exposure dose for a prescribedabsorbed dose at the site of interest is first determined and then,`from the exposure dose rate, the treatment time is calculated. Thus,the technician is required to calculate the treatment time for eachgiven dose, set this time on a timer and switch on the radiation machinelwhich is automatically switched of at the expiry of the set time byswitching means associated with the timer.

rIhe calculation of the treatment time is carried out in accordance withthe characteristics of the radiation machine and with the techniquebeing employed. Radiotherapy techniques include xed field techniques, inwhich the field oi radiation remains stationary, and rotation techniquesin which the source of radiation rotates around the patient. In fixedfield therapy the calculation of treatment time takes into account theintensity of the source, the tield size at the patients skin and thedistance between the source and the patients skin. In rotation therapy,the field size at the skin and the distance between source and skin arenot constant so that the calculation is fundamentally more difiicult;however it may be simplified in practice -by using the concept oftumor-air ratio and so Afar as the technician is concerned thecalculation required for determining the treatment time in rotationtherapy is similar to that for fixed iield therapy except that he isconcerned with tumor-air ratio instead of field size.

Whether a fixed eld or a rotation technique is used the task of thetechnician in making the necessary calculation can be made fairly simpleby the compilation of suitable tables correlating the variables involvedin such manner that no great mathematical skill is demanded of thetechnician. However human error cannot be eliminated. On the other handit cannot be tolerated, for obvious reasons. Therefore it is thepractice in well-run hospitals to require that before treatment iscommenced all such calculations are checked by someone other than thetechnician who makes the original calculation. This is often a greatnuisance but the alternative is to risk a mistake which might haveserious consequences and which would in any event result in a wrongtreatment being administered.

Apart from the difficulty arising from the need to avoid human errorsthe calculation tables with which the technician is provided must beperiodically revised to take into account the decay of the radioactivesource or the change in exposure rate from the X-ray machine.

It is an object of the present invention to provide a device forcomputing the time that a patient should be exposed to radiation from agiven source of radiation, under a specified radiotherapie technique, inorder that he will receive the required exposure dose. For the sake ofbrevity such a device is referred to hereafter as a dose computer.

It is a further object of the invention to provide a dose computer whichcan be adjusted t-o take into account changes in the intensity of thesource of radiation.

The invention provides a dose computer comprising a first rotatabletapered roller, a second rotatable tapered roller, arranged in axiallyparallel side-by-side relationship with said first roller and taperinginopposite direction to said first roller, and coupling means forconnecting said rollers, said coupling means being linearly displaceablebetween said rollers, said rollers being tapered substantially inaccordance with the equation D feo-1 +6-.,

wherein c is a constant, D is the distance between the axes of therollers less the separation between the rollers and j'fx) is the radiusof one of the rollers at an axial distance x from an origin located atthe center of that roller.

One embodiment of the invention is illustrated in the accompanyingdrawings in which FlG. l shows schematically a dose computer;

FIGS. 2 and 2a show the manner in which the tapered rolls of the dosecomputer of FIG. l are mounted;

FIGS. 3 and 3a show means for coupling to each other the tapered rollsof the dose computer of FIG. 1;

FG. 4 is a diagram showing the parameters used in calculating an idealshape for the rollers of the dose computer of FlG. l, and

HG. 5 is a graph showing the maximum percentage deviation fromlogarithmic scale produced by the use of linearly tapered rollers, as afunction of speed range ratio R.

The dose computer' comprises two hardened steel, tapered rollers 1 and 2rotatably mounted in bearings 3 and lon the sides 5' of a U-shaped frame6. The frame 6 has been omitted from FIG. il tor the sake of clarity andis shown in FlG. 2 which omits most of the details of FiG. l. The roller1 is driven at constant speed by a synchronous electric motor 7 througha gear box 8 and gear wheels Si and 10. The roller Z drives a digitalcounter' lil through gear wheels 12 and 13, the gear wheel 13 beingsecured to a rotatable shaft 14 to which is also secured a disc i5carrying a pin 16. The pin 16 engages a slotted arm 17 connected to thecounter 1:1 and rotation of the disc 15 causes a reciprocating movementof the arm 17 which serves to operate the counter 1l.

The rollers 1 and 2 are coupled to each other by two hardened steelballs 18 and .12 which are in contact with the rollers 1 and 2respectively and with each other. For the sake of clarity the manner inwhich the balls 18 and 19 are mounted between the rollers 1 and 2 hasnot been shown in FIG. 1 but appears from FIG. 3 which v is a partialsectional view of the dose computer of FIG.

1 along the line III-Hl of FIG. l. The balls 18 and 19 are accommodatedin a block 2th which is provided with a cylindrical bore 21 having anannular insert 22 of graphite-loaded nylon. The diameter of the insert22 is only slightly greater than that of the balls 18 and 19 so as togive a snug tit while penmitting free rotation of the balls. The lengthof the bore 12d. is somewhat less than the sum of the diameters of theballs y18 and 19 so that each of these balls projects slightly above theface of the block Zit; thus the rollers 1 and 2 contact the balls 1S and19 respectively without contacting the block 20. It may ybe mentioned atthis point that the balls 18 and 19 could v'be replaced by a single ballbut that this is not desirable because movement of the block 20 relativeto the rollers 1 and 2 would then involve sliding of one face of theball on the surface of one of the rollers instead of the rolling actionat both rollers which is obtained by using two balls. Since slidinggives rise to greater frictional acarrear uw forces than rolling theuseful life of the rollers and balls is diminished if only one ball isused. Experience has shown that with two balls the extent of wearing isunappreciable over a period equivalent to two years normal use of thedose computer.

Fl YS. 2 and 2a illustrate the way in which the rollers l and 2 aremounted so as to be in resilient contact with the balls i8 and lil. rTheframe 5 is divided into a movable upper `frame member 5l and a fixedlower trame member 52 hingedly connected by hinge members 53. Coilsprings 54 are secured to the sides 5 of the trame 6 and urge the upperframe member 5l towards the lower `frame member 52. The bearings 3 ofthe roller l are mounted in the upper frame member 51 and the bearings 4of the roller 2 in the lower frame member 52 so the action of thesprings Se urges the rollers l and 2 towards one another. Thereby therollers l and 2 are kept in rolling contact with the balls l and 19.

The block has a threaded bore 23 at right angles to the bore 2l. Acoarse screw 24 passes in threaded engagement through the bore 23 and ismounted in bearings 25 in the sides 5 of the frame 6. The screw 24 isrotatable by means or" a knurled knob 216` fixed thereto and uponrotation serves to move the block 20', with the balls 1S and li?,linearly between the rollers 1 and 2. The block 20 is prevented fromrotating with the screw 23 by means of a lixed rod 27 extending betweenthe sides 5 of the frame 6 and fitting into a recess 27a located on theback -face or the block so that the block 20 slides along the rod 27.

The block 20 carries a pointer 28 that moves over a calibrated scale 29which is one of tour scales carried by a drum 30 rotatably mounted inbearings 30a in the sides 5 of the trame 6. Each of these four scalescorresponds to a given radiotherapy technique. The scale 29 is graduatedin tield sizes ranging from 0 to 400` sq. cm. and is calibrated xedfield therapy with a particular radiation machine using a source topatient distance of 80 cm. The knob 3l by means of which the drum 30 isrotated is marked at four points aligned with the four scales toindicate the nature of the technique to which each scale corresponds. Inthe illustrated embodiment the marking on the knob 3l adjacent the scale29 indicates the source to patient distance of 80 cm. A dierent scalewould be used for operation with a different source to patient distance.Also, one or more of the scales on the drum 30 may be calibrated interms of tumor-air ratios -for use in rotation therapy. Selection of oneof the four techniques corresponding to the four scales on the drum 30simultaneously selects, through a linkage 32, an appropriate gear of thegear box S.

It will be seen that the rollers 1 and 2 and the balls 1S and 19constitu-te the essential parts of a continuously variable speed drivebetween the gear box S and the counter ll, the ratio of which can bechanged by rotation of the knob 26. When using the computer theradiographer merely has to select the appropriate technique by rotationof the knob 3l, bring the pointer 2S to the spot on the scale 29corresponding to the field size or tumor-air ratio involved, set theprescribed dose on the counter 1l by means of the knob 33 and thenswitch on the motor 7, simultaneously switching on the radiationmachine. When the counter has been driven tor such a length of the timethat the prescribed dose has been administered, i.e., the reading on thecounter has been reduced to Zero, a switch 34, such as is shown in U.S.Patent No. 1,992,841, to O. C. Roesen, associated with the counter isopened and breaks the circuit of the motor 7, simultaneously shuttingoff the radiation machine.

The dose computer of the invention is calibrated for use with aparticular radiation machine but it is clear that provision has to bemade to take into account the decay of the radioactive source or achange in output of the machine. For this purpose there is provided ascale 3S vfixed to the block 20. The pointer 23 is carried by slottedplate 36 which is secured to the scale 35 by means of two screws 37passing through the slot in the plate 36 and engaging in threaded holesin the scale 35. By a simple screwdriver adjustment the plate 36 andtherewith the pointer 28, may thus be displaced relative to the scale 35so as to change the set distance between the pointer 23 and the balls 18and 19.

The operation of the device depends upon the scales carried by the drumSti being logarithmic. Suppose the output rate of rotation W0 is relatedto the input rate of rotation Wi by WozWieCX where x is the distance ofthe balls 18 and 19 from a reference point and c is a constant; then ifone wishes to change the output speed by a factor eey, one merely -movesthe pointer 28 a distance y relative to the balls i3 and 119 to yieldThus corrections `for decay or change in output may be made with onesimple adjustment. Any other type of scale would require theconstruction of a new tield size scale or tumor-air ratio scale everytime the exposure rate from the machine altered. Thus the rollers 1 and2 of the dose computer should be designed so that Let the required shapeof roller be represented by f(x) as indicated in FIG. 4 with the originat the centre and the distance between the rollers axes Dl-A. Then theradii at distance x are f(x) and D-f(x) and V 0 fte) Wi D-f(x) lt isdesired that this ratio be eCX. Solving there is obtained D ful-m (l) Itwould `be very dillicult to machine rollers with a taper given byEquation 1 so it was decided to investigate the possibility of a lineartaper with f(x)=al-bx. The ratios WO/Wi lfor the two expressions areThese may be made equal at x=0 and X=+x0 by the proper choice of a andb. When this is done it is easily seen that they match at x=-x0. Themaximum deviation between the two expressions may be shown to occurwhere x is approximately itlrxo. At this value of x the differencesbetween the two expressions for W0/ W1 of Equation 2 have beencalculated and expressed as a percentage of the exponential expression.This deviation is plotted in FIG. 5 as a function of the overall rangeof speeds, R, from end of the roller to the other. For speed ratios lessthan 4:1 the maximum difference is less than 1 percent.

In the embodiment described it was found convenient to have a speedratio of about 2:1 from one end of the roller to the other. The actualratio is indicated on the auxiliary scale 3S which ranges from 1.0 to0.60. When R=2 the maximum difference is 0.13 percent which is entirelysatisfactory `for practical operations.

It will be appreciated that if the embodiment described were intendedfor use in connection with only one radiotherapy technique the gear box8 and the drum 30 could be dispensed with.

When the dose computer has been in use for a considerable length of timethe whole of the decay adjustment provided by the scale 3S willeventually become used up. However it is then only necessary to changethe ratio of the gear wheels 12 and 13 in order to make it possible toreturn the pointer 2S to its initial position with respect to the scale35.

We claim:

1. A dose computer for a radiation machine comprising a first rotatabletapered roller, a constant speed motor for driving said iirst rollerthrough a gear box at one of a plurality of constant speeds, a secondrotatable tapered roller arranged in axially parallel side-bysiderelationship with said rst roller and complementary thereto, a counterdriven by said second roller, coupling means for connecting saidrollers, said coupling means being linearly displaceable between saidrollers and com prising a pair of balls in rolling contact with eachother and in rolling contact with said rollers, means for setting saidcoupling means at a desired distance from the ends ot said rollers, anindicator carried by said coupling means, a logarithmic scale calibratedin accordance with the characteristics of said radiation machine and achosen radiotherapy technique, said indicator being movable over saidlogarithmic scale, the radii of each of said rollers being determined bythe equation wherein c is a constant, D is the distance between the axesof the rollers less the separation between the peripheral faces of therollers and f(x) is the radius of one of the rollers at an axialdistance x from an origin located at the centre of that roller.

2. A dose computer according to claim 1 wherein said coupling meanscomprises a block secured against rotation and having a bore of suchdiameter as to accommodate said pair of balls with each of the ballsprojecting slightly beyond a face of the block to contact one 6 of saidrollers, said block also having a threaded bore at right angles to saidfirst-mentioned bore, and a rotatable coarse screw passing in threadedengagement through said threaded bore whereby rotation of said coarsescrew effects linear displacement of said block between said rollers.

3. A dose computer according to claim 1 which further comprises a firstframe member and a second frame member pivotally connected to said rst`frame member and wherein one of said rollers is mounted in said rstframe member and the other of said rollers is mounted in said secondframe member and means are provided for resiliently urging said secondframe member towards said rst frame member whereby said rollers are heldin contact with said balls.

References Cited in the iile of this patent UNITED STATES PATENTS558,315 Shaw Apr. 14, 1896 755,084 Vermehren Mar. 22, 1904 825,363Vermehren July 10, 1906 1,992,841 Roesen Feb. 26, 1935 2,159,882 BordenMay 23, 1939 2,206,252 Gould July 2, 1940 2,602,338 Opocensky July 8,1952 2,731,831 Schaefer Jan. 24, 1956 2,801,795 Williams Aug. 6, 19572,872,814 Brown Feb. 10, 1959 2,979,256 Cushman Apr. 11, 1961

1. A DOSE COMPUTER FOR A RADIATION MACHINE COMPRISING A FIRST ROTATABLETAPERED ROLLER, A CONSTANT SPEED MOTOR FOR DRIVING SAID FIRST ROLLERTHROUGH A GEAR BOX AT ONE OF A PLURALITY OF CONSTANT SPEEDS, A SECONDROTATABLE TAPERED ROLLER ARRANGED IN AXIALLY PARALLEL SIDE-BYSIDERELATIONSHIP WITH SAID FIRST ROLLER AND COMPLEMENTARY THERETO, A COUNTERDRIVEN BY SAID SECOND ROLLER, COUPLING MEANS FOR CONNECTING SAIDROLLERS, SAID COUPLING MEANS BEING LINEARLY DISPLACEABLE BETWEEN SAIDROLLERS AND COMPRISING A PAIR OF BALLS IN ROLLING CONTACT WITH EACHOTHER AND IN ROLLING CONTACT WITH SAID ROLLERS, MEANS FOR SETTING SAIDCOUPLING MEANS AT A DESIRED DISTANCE FROM THE ENDS OF SAID ROLLERS, ANINDICATOR CARRIED BY SAID COUPLING MEANS, A LOGARITHMIC SCALE CALIBRATEDIN ACCORDANCE WITH THE CHARACTERISTICS OF SAID RADIATION MACHINE AND ACHOSEN RADIOTHERAPY TECHNIQUE, SAID INDICATOR BEING MOVABLE OVER SAIDLOGARITHMIC SCALE, THE RADII OF EACH OF SAID ROLLERS BEING DETERMINED BYTHE EQUATION